1. Field of the Invention
This invention relates generally to relay-type communications satellites, and more particularly to a space division multiple access (SDMA) system which utilizes a pair of satellites having a communcation link between them and each satellite includes directional spotbeam antennae, and an on-board switching matrix.
2. Description of the Prior Art
Conventional space division multiple access (SDMA) communications satellite systems employ a single satellite which includes multiple transmit/receive directional spotbeam antennas. In prior art SDMA systems, several earth stations, each in a different geographical zone access different spotbeam antennas aboard a satellite. Typically, each such antenna communicates with a different geographical zone on the earth's surface. In one prior art system disclosed in U.S. Pat. No. 3,711,855 (entitled "Satellite On-Board Switching" issued to Schmidt et al. on Jan. 16, 1973 and assigned to the assignee of this invention) the satellite contains a switching matrix which interconnects antennas into pairs for specified intervals according to a pre-established sequence so that short bursts of digital information may flow from a transmitting earth station in view of one antenna to a receiving earth station in view of another antenna.
The period of time during which a complete sequence of bursts from all the stations in the system are received at the satellite is designated as the satellite time frame. Such prior art systems require that all participating earth stations be time synchronized with respect to the common satellite time frame so that their respective bursts are simultaneously received at the satellite. In addition, in order to properly distribute the received communications burst the switching matrix on-board the satellite must be precisely synchronized with the satellite time frame so that the proper communications path is established.
Several prior art systems exist for synchronizing SDMA communications satellite systems which utilize communication satellites that do not have a communications link between them. According to the method disclosed in the aforementioned U.S. Pat. No. 3,711,855, a synchronizing burst is transmitted from a designated reference earth station to a satellite relay where detection of the synchronizing burst is used to synchronize the satellite on-board switching matrix. The synchronizing burst is also retransmitted to all earth stations through the on-board switching matrix. In addition, each earth station transmits a characteristic unique word pattern. The unique word patterns are received at the satellite and retransmitted through the on-board switching matrix to the originating earth station. In order to maintain synchronization, each earth station adjusts its time of transmission so that its unique word pattern is received a predetermined time after receipt of the synchronization burst from the reference station.
Another prior art technique for synchronizing a single satellite (SDMA) communications systems is disclosed in U.S. patent application Ser. No. 128,396 (filed on Mar. 26, 1971 entitled "Frame Synchronization Technique for Satellite On-Board Switching Systems" by Shimasaki et al., and assigned to the assignee of the present invention). The synchronization technique disclosed therein is similar to the above-described technique of the Schmidt et al. patent except that instead of using a reference burst transmitted by a designated reference earth stations as the basis for synchronization, system synchronization is controlled by a master oscillator on-board the satellite. Each earth station as in the Schmidt et al. system transmits a characteristic unique word pattern. At the start of every communications frame the on-board master oscillator causes the on-board switching matrix to connect the respective input spotbeam antenna of each station to that station's output spotbeam antenna so that each earth station receives its own unique word pattern. The earth stations maintain synchronization by observing the retransmitted unique word pattern to see if a predetermined portion of the unique word pattern is properly received. If the predetermined portion of the unique word pattern is not properly received, the station will adjust its time of transmission until it is properly received.
The synchronization systems of these and other prior art communications satellite systems can not be used to maintain synchronization in a SDMA communications satellite system which uses two satellites with a communications link between them. This is because such systems in addition to requiring that synchronization be maintained between the on-board switching matrix of each satellite and that satellites associated earth stations, also require that synchronization be maintained between the satellites so that communication may flow from an earth station in view of one satellite through the switching matrices of both satellites to an earth station in view of the other satellite and vice versa.
In a two satellite communications system having a direct inter-satellite communication link in which each satellite includes an on-board master oscillator to maintain synchronization of its on-board switch and associated earth stations, synchronization error between the two satellites may result from relative drift of the satellites respective master oscillators or from improper propagation delay in the inter-satellite communication path due to distance variation between the satellites. Prior art synchronization systems fail to provide a technique for correcting this error.